JPH0473609B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a molecular beam epitaxial growth method, and particularly to a method for improving crystal defects that occur during device fabrication.

(Prior Art and its Problems) Recently, a molecular beam epitaxial growth method (hereinafter abbreviated as "molecular beam growth method") has been attracting attention as a crystal growth method for forming an ultra-thin film structure on a semiconductor substrate. In this growth method, molecular beams are irradiated onto a substrate heated to a high temperature under a high vacuum to cause crystal growth.

FIG. 3 is a sectional view of the main parts of the molecular beam epitaxial growth apparatus. Here 21 is the growth room, 2
2 is a vacuum pump section, 23 is a substrate holder, 24, 2
5 and 26 are molecular beam sources, for example, 24 is an arsenic (As) molecular beam source, 25 is a gallium (Ga) molecular beam source, and 26 is an aluminum (Al) molecular beam source. is maintained.
To explain the growth process using GaAlAs growth on a GaAs substrate as an example, first, the GaAs substrate 27 is fixed to the substrate holder 23, and then the temperature of the substrate 27 is raised. When the crystal growth temperature is reached, Ga, Al,
When the shutters of each As molecular beam source are opened and the substrate 27 is irradiated with molecular beams, GaAlAs crystals will grow.

When manufacturing semiconductor devices using this molecular beam growth method, sufficient care is taken to avoid crystal defects, which are one of the factors that degrade yield and device characteristics. However, it has already become clear that crystal defects always occur in the following cases.

[When the growing crystal surface is exposed to the air or a low vacuum atmosphere] For example, after crystal growth (first growth) is performed on a substrate using molecular beam growth, and processes such as diffusion and ion implantation are performed. Depending on the device structure, it may be necessary to perform molecular beam growth (second growth) on the wafer again. In order to perform such processes such as diffusion and ion implantation, it is usually necessary to expose the material to the atmosphere or a low vacuum atmosphere and perform mask processing and the like. Therefore, since the second growth is performed after completing this process, there are many crystal defects near the interface between the crystal surface exposed to the air or low vacuum atmosphere and the growing crystal above it. arise.

[When crystal growth is performed by heating a semiconductor substrate to a high temperature] For example, when crystal growth is performed on a GaAs substrate by molecular beam growth,
When growing GaAlAs, crystal growth is performed by irradiating molecular beams of Ga, Al, and As onto a GaAs substrate heated to high temperature in a high vacuum. However, because the temperature of the GaAs substrate is high, the substrate is The surface thermally decomposes and As evaporates. Therefore, the surface of the GaAs substrate becomes rough, causing crystal defects.

As mentioned above, crystal defects may occur during the crystal growth process, and the following measures have conventionally been taken to improve this problem.

[Measures to improve when crystal defects occur due to exposing the growing crystal surface to the air or low vacuum atmosphere] When performing local diffusion on the growing crystal surface, a diffusion mask is required; There is no way to spread the virus without it. In addition, when performing ion implantation locally, there is also a method in which a focused ion implanter is coupled to a molecular beam growth device, the ions are guided to the focused ion implanter while maintaining a high vacuum state, and then returned to the molecular beam growth device after ion implantation. Although it is being researched, it is inefficient and poses a practical problem.
That is, at present, it is extremely difficult to perform local manual doping without exposing the material to the air or a low vacuum atmosphere. For this reason, it is difficult to avoid contamination of the surface of the growing crystal, but no appropriate measures to prevent this have been proposed so far.

[When crystal growth is performed by heating a semiconductor substrate at high temperature] As an improvement measure in this case, while irradiating the GaAs substrate with an As molecular beam, the temperature of the GaAs substrate is gradually raised to the temperature required for crystal growth (approximately 600°C or higher). The method used is to raise the temperature to a higher temperature and supply an excess As atmosphere to prevent As from evaporating on the substrate due to thermal decomposition.

However, this method also has the following problems.

As molecular beam required to protect the substrate surface
The amount of As must be given in accordance with the substrate temperature, and in particular, the higher the substrate temperature, the greater the amount required.

The As molecular dose required for GaAlAs growth is typically
This is different from the As molecular dose (mentioned in ) required to protect GaAs substrates. That is, usually
The As molecular dose required for GaAlAs growth is lower. Therefore, if a GaAlAs crystal is grown with a high As molecular dose for substrate protection, the quality of the GaAlAs crystal will deteriorate.

As mentioned above, it is difficult to suddenly change the As molecular dose with only one As molecular beam source, so two As molecular beam sources are usually prepared.
A method has been adopted in which the As molecular beam source is switched according to each process, for protection of the substrate surface and for GaAlAs crystal growth. However, preparing multiple molecular beam sources of the same element limits the number of molecular beam sources of other elements, reduces the degree of vacuum in the growth chamber, increases the size of the equipment, and is not economical. That there is a problem from.

As described above, conventionally there has been no method for efficiently improving crystal defects that occur during crystal growth processes such as semiconductor device fabrication using molecular beam growth methods.

(Objectives and Features of the Invention) The present invention has been made in view of the above-mentioned drawbacks of the prior art, and aims to reduce crystal defects that occur during the crystal growth process using the molecular beam epitaxial growth method, thereby improving the yield of wafers or the characteristics of devices. The object is to provide an improved molecular beam epitaxial growth method.

A feature of the present invention is that when necessary crystal growth is performed on a semiconductor substrate using a molecular beam growth method to produce a wafer or a semiconductor element, a polycrystalline film (protective film) made of a compound semiconductor is formed on the surface of the growing crystal. and a step of thermally decomposing at least the protective film immediately before crystal growth is to be performed again on the protective film.

The present invention will be explained in detail below using the drawings.

Example 1 FIG. 1 shows an example according to the present invention, in which GaAs
2 is a diagram for explaining a growth process when growing a GaAlAs layer 2 on a substrate 1. FIG.

Figure 1a shows a GaAs substrate 1 grown in a molecular beam epitaxial apparatus (hereinafter referred to as "MBE apparatus") using a conventional method.
This figure shows the state of the GaAs substrate 1 when it is loaded into a wafer.

Usually, after degreasing and cleaning, the GaAs substrate 1 is treated with H ₂ SO ₄ :
After the surface is etched with an etching solution of N ₂ O ₂ :H ₂ O=5:1:1, rinsed with water, and dried, it is fixed on a substrate holder.
Loaded into an MBE device (not shown). At this time, an extremely thin oxide film 3 of GaAs is formed on the GaAs substrate 1. This oxide film 3 is removed by Ga during the etching, rinsing and drying steps mentioned above.
and As combine with oxygen to form oxides,
It is already known that a mixed crystal of these oxides is formed as the oxide film 3. Note that the oxide film 3
Since it decomposes thermally at about 600 degrees Celsius, it will not have any negative effect on subsequent crystal growth.

Furthermore, as a preliminary preparation, the inside of the MBE apparatus is placed in a high vacuum state, and the temperature of the substrate holder is adjusted to the crystal growth temperature of the protective film 4 (approximately 100° to 400°C) as described below.
raise it to

Next, there are features of the present invention. A protective film 4 is formed on the oxide film 3. Specifically, when the temperature of the basic holder reaches the crystal growth temperature of the protective film 4 (for example, 300°C), the temperature shown in FIG.
A GaAs polycrystalline film, that is, a protective film 4 is formed on the oxide film 3 as shown in FIG.

This GaAs protective film 4 is a GaAs protective film 4 even when the crystal growth temperature of the GaAlAs layer 2 reaches about 700 degrees.
This is provided to protect the surface of the GaAs substrate 1 so that the surface of the substrate 1 does not deteriorate due to immediate thermal decomposition.

In addition, the film thickness when forming the GaAs protective film 4
_tG is not particularly limited, but from the viewpoint of ease of removal of the GaAs protective film 4 described in the next step, it is preferable that it can be removed within a few minutes. for example
If the crystal growth temperature of the GaAlAs layer 2 is 700° C. and the thickness t _G of the protective film 4 is about 500 Å, it can be removed in about 5 minutes. Thickness of this GaAs protective film 4
_tG is related to the protective film removal temperature, that is, the crystal growth temperature T (absolute temperature), and can be expressed by the following approximate expression.

t _G 3×10 ¹⁷ exp (-33000/T) [Å] ...(1) In order to obtain the desired film thickness t _G , the As molecular dose,
The Ga molecule dose and each irradiation time may be adjusted.

After the GaAs substrate 1 is protected by the GaAs protective film 4, the temperature is raised to about 700 degrees, which is the crystal growth temperature of the GaAlAs layer 2, while being irradiated with the As molecular beam, and the temperature is maintained for several minutes (for example, about 5 minutes). )Hold. When this crystal growth temperature (approximately 700 degrees) is maintained for several minutes, the GaAs protective film 4 is removed by thermal decomposition as shown in Figure 1c, and then the oxide film 3 is removed.
can be similarly thermally decomposed and removed, leaving only the GaAs substrate 1.

This state can be observed using a backscattered electron diffraction image, and as the backscattered electron diffraction image changes from a ring-shaped diffraction image to a dot or line-shaped diffraction image, the GaAs protective film 4 and the oxidized membrane 3
can be determined to have been completely removed.

Therefore, by irradiating with Ga and Al molecular beams immediately after the process, the result as shown in Figure 1d is obtained.
A GaAlAs layer 2 can be grown on the GaAs substrate 1 .

By this method, GaAs substrate 1 and GaAlAs layer 2
There are almost no crystal defects on the crystal surface, and good crystal growth can be achieved. Further, since it is sufficient to irradiate only the amount of As molecular beam necessary for crystal growth of the GaAlAs layer 2 from the beginning, only one As molecular beam is required.

In addition, in the above explanation, the case where the GaAlAs layer 2 is grown is explained, but the case where the GaAlAs layer 2 is grown is explained.
It may have a multilayer structure of GaAIAs layers, and can be implemented regardless of the conductivity type or impurity concentration of the substrate and growth layer.

In addition, InGaAs, InAlAs,
When growing a multilayer film made of InGaAlAs or a combination thereof, an InAs protective film or an InP
A protective film may be used.

Furthermore, it is also possible to use a combination of other - group or - group compound semiconductors by using a compound semiconductor layer having a composition that can be easily removed at the crystal growth temperature as a protective film.

Example 2 A case in which the surface of a growing crystal, which is one of the causes of crystal defects, is exposed to the air or a low vacuum atmosphere will be explained using the manufacturing process of a static induction transistor as an example.

FIG. 2 shows another embodiment according to the present invention,
FIG. 2 is a diagram for explaining a growth process when manufacturing a static induction transistor using GaAs.

FIG. 2a shows an n ⁺ -GaAs layer 6 and an n ^- - layer grown on an n ⁺ -GaAs substrate 5 by a conventional molecular beam growth method.
This is a crystal grown GaAs layer 7. (First crystal growth). Conventionally, the wafer on which the first crystal growth has been completed is then taken out into the atmosphere or low vacuum and subjected to an impurity diffusion or ion implantation process, but in the present invention, the following process is performed before that.

During the first crystal growth, the growth chamber temperature was 600℃.
The protective film crystal growth temperature (approximately
The temperature in the growth chamber is lowered to about 100 to 400°C, and the polycrystalline film of GaAs or InAs, which is a feature of the present invention, that is, the protective film 4 is grown as n ^- -GaAs as shown in Fig. 2b.
Formed on layer 7.

Here, whether to use the GaAs protective film 4 or the InAs protective film is determined by the second crystal growth temperature in the step described below. this is
This is because the GaAs protective film 4 and the InAs protective film have different thermal decomposition temperatures, and as will be described later, this protective film 4 must be removed by thermal decomposition. Therefore, it is better to use the InAs protective film when the second crystal growth temperature is 700 degrees Celsius or lower, and use the GaAs protective film 4 or the InAs protective film when the second crystal growth temperature is 700 degrees Celsius or higher.

Further, the thickness of each protective film is also related to the second crystal growth temperature, etc., but the thickness does not limit the present invention. For example, if the crystal growth temperature is 700
If the thickness of each protective film 4 is about 500 Å, it can be removed in about 5 minutes. As mentioned above, the thickness of this protective film varies depending on the crystal growth temperature, that is, the protective film removal temperature, and the approximate formula for the film thickness is as follows.

Thickness t _G of the GaAs protective film is the same as equation (1).

Film thickness of InAs protective film t _I t _G 1×10 ¹⁷ exp (-29000/T) [Å] ...(2) The wafer with protective film 4 is taken out from the molecular beam growth chamber into the atmosphere, and the silicon nitride film is After the silicon nitride film 5 is formed on the protective film 4, desired portions of the silicon nitride film 8 are removed by photolithography.
Using the silicon nitride film 5 as a mask, for example, cadmium (Cd) is applied to the protective film 4 as shown in FIG.
and diffuses to the GaAs layer 7 to form a Gd diffusion region 9.

In this case, conventionally, the crystal surface of the GaAs layer 7 was exposed to the atmosphere, which caused crystal defects, but as in the present invention, the crystal surface of the GaAs layer 7 is protected with the protective film 4 and then exposed to the atmosphere. Since the surface of the GaAs layer 7 is taken out, it is possible to protect the surface of the GaAs layer 7 from the adverse effects of direct exposure to the atmosphere.

Next, silicon nitride film 8 is removed using a conventional method. Therefore, as shown in FIG. 2d, the protective film 4
is exposed to the atmosphere again.

The wafer from step 2 is returned to the molecular beam growth chamber in a high vacuum, raised to the second growth temperature, and held for about 5 minutes after reaching the growth temperature. As a result, the protective film 4 is thermally decomposed at the growth temperature and completely removed. At this time, the state of removal of the protective film 4 can be determined by the change in the reflected electron beam diffraction pattern from an annular pattern to a linear pattern.

Immediately after the process, by conventional methods.
By sequentially crystal-growing the n ^- GaAs layer 10 and the n ⁺ -GaAs layer 11 (second time), a static induction transistor having a structure as shown in FIG. 2e can be manufactured. Note that the n ⁺ −GaAs substrate 5
is the source, cadmium diffusion region 9 is the gate,
The n ⁺ -GaAs layers 11 correspond to drains, respectively.

As described above, in the present invention, a protective film 4 is provided on the crystal surface before the wafer is taken out into the atmosphere, and when the wafer is returned to the high vacuum atmosphere, the protective film 4 is removed and the molecular beams of the required semiconductor layer are immediately removed. The generation of crystal defects is prevented by epitaxial growth.

Note that although the above explanation uses a static induction transistor as an example, the manufacturing method is not limited to this static induction transistor. It goes without saying that any molecular beam epitaxial growth method including a manufacturing process in which the crystal surface is exposed to the air or a low vacuum atmosphere can be applied.

In addition, in this example as well, when growing an n ⁺ -GaAs layer on an n ⁺ -GaAs substrate, it is better to provide the GaAs protective film 4 before growing the crystal, as explained in Example 1. Needless to say.

In addition, the scope of application is exactly the same as in Example 1,
Multilayer structure of GaAs and GoAlAs on GaAs substrate,
A device using a multilayer film of InGaAs, InAlAs, InGaAlAs, or a combination thereof on an InP substrate (However, in this case, the protective film is InAs or InP
It is preferable to use ), crystal growth using a combination of - group or - group compound semiconductors other than the above, and regardless of the conductivity type of the crystal growth layer.

(Effects of the Invention) As explained above, the present invention provides a method for removing a protective film on a substrate or wafer during crystal growth on a substrate or when exposing a wafer to the air or a low vacuum atmosphere, which can cause crystal defects. By once growing the crystal and then performing the next step, a device with extremely few crystal defects can be manufactured. Therefore, the present invention can improve yield reduction and deterioration of device characteristics due to crystal defects caused by conventional techniques, making it possible to fabricate highly reliable devices and reducing the number of molecular beam sources in the molecular beam epitaxial growth apparatus. The effect is extremely large as it is economically viable.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view for explaining the process of crystal-growing a GaAlAs layer on a GaAs substrate as an embodiment of the present invention, and FIG. 2 is a schematic cross-sectional view of another embodiment of the present invention. FIG. 3 is a schematic cross-sectional view for explaining the manufacturing process of an electrostatic dielectric transistor, and FIG. 3 is a schematic cross-sectional view showing the main parts of the molecular beam epitaxial growth apparatus used in the present invention. 1...GaAs substrate, 2...GaAlAs layer, 3...
Oxide film, 4... protective film, 5... n ⁺ −GaAs substrate,
6...n ⁺ -GaAs layer, 7...n ^- -GaAs layer, 8...
...Silicon nitride film, 9...Cd diffusion region, 10...
...n-GaAs layer, 11...n ⁺ -GaAs layer, 21...
...Growth chamber, 22...Vacuum pump unit, 23...Substrate holder, 24, 25, 26...Molecular beam source, 27...
…substrate.

Claims (1)

[Scope of Claims] 1. In a molecular beam epitaxial growth method in which a semiconductor substrate is irradiated with a molecular beam to grow at least one layer of semiconductor crystal, a compound semiconductor polycrystal having a thermal decomposition temperature near the growth temperature of the crystal. a first step of forming a layer as a protective film on the substrate or an uppermost semiconductor layer formed on the substrate in a state where the substrate or the uppermost layer is at a temperature below the growth temperature of the crystal; a second step of increasing the temperature of the top layer to the crystal growth temperature; a third step of removing the protective film by thermal decomposition by maintaining the crystal growth temperature; and the third step. a fourth step of growing a subsequent semiconductor layer immediately after removing the protective film. 2. The molecular beam epitaxial growth method according to claim 1, wherein the protective film is formed on the substrate. 3. Claim 1, characterized in that the protective film is formed on one semiconductor layer formed on the substrate or on the uppermost layer of a plurality of semiconductor layers formed on the substrate. Molecular beam epitaxial growth method.